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Why aren't swarms of cheap consumer-level drones a viable anti-aircraft tactic?
In the Russia/Ukraine war, when it comes to cheap, consumer-level drones, I've only heard of them used for reconnaisance, not for anti-aircraft defence. But given that:

In civil aviation, a single rogue drone is often sufficient to shut down an airport due to the danger it poses
Bird strikes are very dangerous, and presumably a drone strike is no different
Large swarms of drones can be relatively easily controlled by modern software to form coordinated patterns
The drones are cheap (in the hundreds of dollars each), and even if one drone scores a strike the rest can still be reused
The drones can carry other payloads, e.g. small explosives, always-on cheap consumer laser pointers, liquid gallium to weaken the airframe if they score a hit, etc

... I would expect swarms of cheap drones to be a very effective way of interfering with invading aircraft, if not planes then at least helicopters. Yet one never hears of such tactics being used - why?

Like aviation equivalent of landmines? Then you run into all the difficulties of landmine tactics.

In WW1, they used balloons with hanging steel cables to do something similar. A famous American WW1 Fighter Pilot's (Frank Luke), claim to fame was as a "Balloon Buster" who shot down and destroyed German balloons. Luke AFB in Phoenix is named for him.

Liquid gallium? at only $50 for 50 grams (on amazon), yeah that is real cheap. Also, it will do nothing to painted surfaces of the aircraft and likely nothing to exposed aluminum that has developed a protective oxide.

@GlenYates [Citation Needed]. I've seen people in YT melting aluminum padlocks with gallium straight out of the package. We can reasonably assume these (civilian-grade) padlocks had either painting or protective oxide layer. Though forming such layer may take anywhere from 1 month to 1 year dependent upon conditions. http://www.cabuk1.co.uk/FAQ%207%20Oxidation%20of%20aluminium.pdf

Regardless if it melts or not, I still doubt the effectiveness of splashing liquid metal on fast-moving aircraft as a valid combat tactic.

@Mindwin Here is a youtube video showing gallium doing absolutely nothing to aluminum cans, unless the surface of the can is first scuffed up with sandpaper.

A high-speed collision with a drone might do more damage to the outer paint/oxide layers than sandpaper...

this question reminds me of https://en.wikipedia.org/wiki/1999_F-117A_shootdown  .  maybe if you knew a plane was flying a fixed path at a regular time every day you could pull it off if you got extraordinarily lucky, but it's not a strategy.

Regarding the risk presented by a single drone or bird strike: what is considered unspeakably dangerous in a civilian commercial setting is simultaneously laughably ineffective in a military context. One out of a hundred airliners crashing is a tragedy; one out of a hundred bombers crashing is being bombed back to the Neolithic instead of the Mesolithic.

Civil airports can be shut down by drones because they can't use radio direction finding to trace your drone's control signal and then blow the surrounding hundred meters to kingdom come. At least, not without a lot of paperwork.

I think we will see swarms of small drones used at some point, but probably against slower-moving targets (such as people or land vehicles) rather than aircraft.

arent drone (especially those used for swarm) a lot slower than fighter jets? I feel looking at the speed of fighter jets, something that can take down fighter jet will take a form of missile... may be swarm of missiles?

Swarms of cheap drones would a viable anti-aircraft tactic, depending on the size of the swarm and the User's idea of "cheap" or "consumer-level."
All the technical objections so far Posted here are dwarfed by cost or number; certainly by both.
NATO's most-used F-16 costs from $30-60 million.
Stick with the $30M and if your drones cost $1,000 each, cost equivalence would allow 30,000 drones per F-16.
Your only problem would getting the drones into the target's path.

Maybe of interest here: the Pom-Pom AA-Arty, which is mounted on the HMS Belfast on the Thames. It was meant to put an umbrella of fine shrapnel around the ship to deny planes to come near it.

Ukraine has used consumer grade drones for far more than reconnaissance; they're used to attack armored vehicles, troops, and artillery by dropping grenades on them. So they could certainly be fitted with explosives to increase their lethality against an aircraft.

If a drone has a one-in-five-thousand chance of bringing down an aircraft, that makes it a serious danger on a civilian airport, but useless as a weapon for attacking military planes. And if you want to increase your chances by flying a thousand of them, then you need to think about how to control such a swarm, which is going to cost a lot more than the price of the cheap consumer drones.

Drone swarms are pretty much a consumer product these days. They are used to produce aerial light shows.

@DrMcCleod no they are not. They are not an off-the-shelf product.

@Jpe61 I didn't say 'off the shelf', I said 'consumer', like anything else that offers wet-hire: https://www.swarmtechdrones.com/

@DrMcCleod You are using the word "consumer product" very differently than most people use it.

Well I guess they are in a consumer product the sense that any consumer with considerable finances can order this service to put up a lightshow (or air defense) @barbecue

Great. But what advantage does this offer compared to just designing a missile with an expanding rod fragmentation warhead to take out an aircraft?
While your comments are true that small drones have caused damage or even total destruction of a manned aircraft, keep in mind these accidents generally occur at low altitudes, typically less than 400 feet (130 m) in the vicinity of airports and the collisions are pretty rare as compared with the total number of flights which end safely every day.
Drones also have an absurdly limited operating envelope, as a compare to guided anti aircraft missile, which means aircraft can simply climb above an area if there is a perceived threat of collision with these things.
Typically anti-aircraft defenses employ a large screen of shrapnel emitted in the vicinity of an aircraft in order to destroy it. It is much cheaper to design a warhead that way, than build a swarm of drones to try and do the same thing.

Beg to differ on limited range. (Just make it a little bigger).  Just as in space exploration, there is huge weight savings eliminating the human and human required support systems.  Add to that the elimination of biological G load limitations.  This is nothing new.  Missiles have been chasing airplanes for years.  The drones simply replace the A-10 for tactical air support and interdiction of supply convoys.

@RobertDiGiovanni 1) even the best drones (quadcopters) are limited in terms of speed range an endurance.  Current guided AA missiles have ranges in excess of 200 nm depending on type and max altitudes exceeding 100,000 feet.  Second, what’s the point in designing a swarm of autonomous drones as compared with operating an SAM site under autonomous or AI control?

Yes, perhaps I should say with some modifications.  I see drones as eyes, supporting an AAM site under autonomous or AI control, in other words a "carrier" drone that is armed (ground missiles can certainly be integrated in too).  My thought is drones spotting for a modified ground to ground HAWK battery.  I just can't figure out if it should be horse-drawn or with oxen.

@CarloFelicione Great points, thanks, but I guess I wasn't super-clear in my question: I have no doubt of course that a system specifically designed to take down aircraft will do a better job than a swarm of drones. But missiles can be hard to come by (see all the recent discussion and controversies of whether or not to supply certain types of weapons to Ukraine, etc). Drones can be bought off-the-shelf, without raising eyebrows, cheaply. I was wondering if they could be a cheap, readily-available alternative in the absence of missiles. But I see your point about the operating envelope.

@EugeneOsovetsky Thing is, you would need dozens if not a hundred drones to be as effective as one missile, plus a whole bunch of people to operate them. Light anti-air missiles (MANPADS) have been flowing to Ukraine ever since the beginning of the war. All the controversies are mostly about tanks, fighter jets, and grown-up anti-air systems (the kind with a battery of launchers, a big radar and a command centre) and there's no way you could substitute those with a swarm of drones.

I'm gonna venture a guess that development cost for a missile system is in the order of magnitude of hundreds of million of dollars. Missile unit cost is at least tens of thousands. Not really comparable with drones at all.

@Jpe61 but what good is a cheap drone if it can’t do the job for the reasons stated above?

It is not, but I think the comparison is futile. Cost difference of developement, acquiring and operating these two systems is at least three orders of magnitude. I highly suspect the acievable kill ratio of drone swarm anyway.

Modern attacking aircraft can launch Air-to-Ground missiles from many miles away. How many cheap drones would need to be manufactured to effectively sanitize a block of airspace 50,000 feet high over thousands of square miles of territory?

" true that small drones have caused damage or even total destruction of a manned aircraft" This is, as far as I can tell, false. What incident are you thinking of where a drone destroyed a manned aircraft?

@Harabeck https://petapixel.com/2018/02/16/drone-causes-aircraft-crash-first-time-us-report/

@CharlesBretana I think you've got the key right there.

@FreeMan well that, but it’s really about the performance, cost effectiveness, complexity, and practicality of such a system.  Good engineering, not to mention the realities of war, strongly favor the KISS principle or Keep It Simple, Stupid.  Trying to fill up that volume of airspace with an armada of complicated drones versus a guided rocket fitted with a blast fragmentation warhead just makes for an expensive, complicated and troublesome weapon system that has little benefits over conventional alternatives.

@CarloFelicione that was my point - trying to fill the airspace with what would effectively be self-propelled AA chaff would be difficult and expensive, at best.

Note, also, that an attacking aircraft is going to be moving far faster than any drone.  Thus any attack by drones must be based on figuring out where the attacking plane is going to go and already being there.  If you can do that you can also put somebody behind a bush with a MANPAD and be far more likely to actually down the aircraft.

Payload. Inexpensive quadrotors have a very limited carrying capacity. I have a HolyStone HS720E. ~$300. A measured 200 grams lifting capacity, and that with reduced control in flight, and much reduced range and speed.
Coordination. Getting several of them up into the path of something is non-trivial.
Still new. Eventually this may be a thing. But not just yet.
(added)
Altitude and range.
Go above 300-400 feet, precise control and positioning becomes a LOT harder. You cannot see your own little UAV. The aircraft you're trying to bring down is only at that level near his runway/airbase. So you need to be near that as well. On his land.

Is precise positioning (assuming the drones can't use GPS and a pre-planned route) more effective than a pilot knowing that the things are up there somewhere, and all-but-invisible? I suppose that a proven destruction of an aircraft by a drone would be needed first for the pilots to take them as a serious distraction or danger.

Theoretically, payload isn't necessary, after all, geese don't carry a payload and they brought down US1549. I think the OP's premise is that if a fighter were to ingest one or two drones, it would cause engine damage that would bring the plane down (or at least cause it to turn back if it's now running on one engine instead of two). Sure, explosives in the drone would help, as would a heavier drone. Beyond that, I agree 100% with your answer: +1.

@FreeMan - I only included the payload info because that was in the original question. But yes...putting a kilogram or so of drone down the intake of a jet WILL give someone a bad day.

@AndrewMorton No more so than knowing that birds are up there somewhere keeps planes grounded now. In order to be a serious threat you need them to be thick enough to have a good chance of hitting an incoming plane, and they need to not run into each other all the time. It's not an unsolvable problem but it does cut against the premise of a cheap, COTS air defense solution.

Your last paragraph is incorrect, most Russian flight ops are very low - below 500’ AGL and often below 200’. First to  go "swoof" past people with Stingers so they can't get it shouldered before you're over their horizon. Second to stay "under the radar" of truck mounted medium and long range SAMs.  Check out Grim Reapers on Youtube for more on what it takes to survive operations in a SAM umbrella, e.g. their simulated attack on Belogrov. So yeah, drone-low.  Network them so three see/hear the enemy and the others position to intercept. Or simply tell the Stinger guy to get ready.

@Harper - Reinstate Ukraine - As ISR platforms, a fleet of little commercial quadrotors would be great. As anti air, not so much. Too slow to effectively get into the path of an oncoming enemy aircraft. Especially if that enemy is coming in at 200'. I suppose you could have an outer and inner ring. The outer to tell the inner ones a jet is coming, and to position themselves in its path. But really....just notify the guy with the Stinger tube.

Consumer drones are small and very slow, so the only viable way to use them for anti-aircraft purposes would be to have a swarm of them covering a certain area, thick enough that an airplane would fly through the cloud and be damaged by the impacts. Military aircraft are built tough so there's no guarantee this would work, but let's assume it would.
The issue is that you need a lot of drones to make that work, and I mean a whole lot. A Stinger missile has an effective range of close to 4km (allegedly, it's probably less than that). Doing a Fermi approximation, let's assume you want to replicate the coverage of a Stinger, so a radius of 3000m, assuming a spacing of 1m between drones. The area of half a sphere is 2πr2, plugging in a radius of 3000m you would need about 56,550,000 drones. Say you could make them explode when an airplane comes close and only need one per every 4 sq meters, then you only need 14 million drones. Say they cost \$300 per drone, that's \$4.2 billion to cover that area, as opposed to a Stinger missile which costs $38,000. Even if my figures are an order of magnitude off it's still an incredibly expensive system. Even if it's just hundreds of thousands there's still a huge logistical challenge to transport, deploy and maintain such a system.

MathJax to the rescue in messing up your formatting! I escaped the $ with a backslash \  and that fixed it. Not sure why only 2 of the 3 $ were messing things up.

@FreeMan it transforms the text between 2 dollar signs, so whenever there is a odd number of them in an answer, the last $ is ignored. (which is why I don't need to escape it in this comment)

You don't need to space them anywhere near that tightly.  Your goal is to get the drone sucked into the engine intake.  That intake draws air in from a wide radius around it, so spacing drones ten meters apart should still give a reasonable kill rate while only requiring a half-million drones or so.

@Mark That's a generous definition of "only"!

@Mark only when stationary or slow. During cruise flight the stream tube is more or less the same size as the intake.

I really don't think so @Mark, 10 meter spacing isn't going to do it. Plus, who says  sucking in one drone is going to take out an airplane?

You're way overstating the Stinger's sky control--if you fire one at an aware target anywhere near max range you just wasted a missile.  The point remains valid, though.

Possibly - but you'll need to pull the enemy down to the effective altitude of cheap drones.  They are already doing that.
Deny the high sky, part 1: high altitude.
The preferable method is to use long-range, long-reach, very large SAM systems with 200-300km range, well in the rear.  The extant systems of this type are huge, "4 missiles per semitrailer + separate RADAR trucks + separate command post" type deals; the missiles are 0.8m diameter and 10m long, basically flying telephone poles. This includes the Soviet S-300 and successor S-400 systems (no NATO equivalent), and warship-based S-300 and AEGIS/SM-2 system.  These control the high sky, but cannot control the middle and lower sky because they are positioned well in the rear, and lower altitudes are below the system's horizon.
In the Ukraine theater, Ukraine and Russia both possess the S-300 truck mounted system, Russia has the evolved S-400, and there are no warships with such systems anymore.
The less preferable method is to exhaust your adversary's supply of precision guided bombs and missiles by making it necessary, either by absorbing the hits to valuable infrastructure, or by using disinformation tactics to make them waste precious precision-guided weapons on bogus targets.  I'm figuring on quite a lot of the latter, given Ukraine's demonstrated history of doing a lot with a little.  Once they are down to World War II tier dumb bombs, to hit a given target their bombers must come well below 20,000 feet (6000m).
Deny the high sky, part 2: medium altitude.
Next, the objective is to deny the medium altitudes below 20,000 feet (6000m).  This can be done several ways. First, use medium SAMs such as Buk, Tor or Pantsir.  By putting everything - RADAR, command and missiles on the same truck, there's a downside in missile size, which limits range to medium - but a massive gain in agility, so they can "keep moving" to avoid counterattack. As such, they can be much farther forward with less of a horizon.
Second, use MANPADs like Stinger or SA-7 which have far shorter range (under 10,000'/3000m altitude) but are yet more portable, and more importantly use infrared seekers, so anti-RADAR missiles don't work on them, and it won't light up the threat-warning receiver on aircraft. The aircrafts' only warning is seeing the puff of smoke. And flares don't work if you don't deploy them.  The MANPAD shooter has an effective field-of-view of "the tree line", buildings or other obstacles, so their access to the sky is somewhat less than 180 degrees.  As such, the aircraft flight path is in that arc only for a limited time.  The higher the aircraft flies, the longer it is in that arc and thus subject to attack. This amount of time matters. because MANPAD shooters take a moment to grab the weapon and get ready.
An aircraft operating in these zones is now denied the lower "good bombing" altitudes of 300-20,000 feet (100-6000m).  So to manage both threats, the aircraft must fly very low.  It defeats the medium range SAMs by dropping into the RADAR "ground clutter", literally below the RADAR.  It defeats MANPADS by shortening the time it is in the shooter's field of view to where there is not time to alert, get to the MANPAD, shoulder up, power up, aim and shoot.  But, this has the effect of denying the enemy medium and low altitudes, and forces them to hug the tree line.  If you don't want to die, you better like the taste of leaves in your teeth.
In the Ukraine theater, everybody has these systems.  On March 29, 2022, Ukraine claims not to have held a clinic in Belogrod on penetrating a modern, networked SAM system.
To see these systems at play in simulation, with discussion, check out the "Grim Reapers team" video on Youtube where they attempted to "fact check" the Belgorod raid. The conclusion that if you fly "not perfect" the SAMs get you, but a more experienced player (who really liked the taste of leaves) made it in and out (even overflew a SAM site too low and close for it to engage).

Let's change the meaning of "drone", though.
I know "quadcopter" is synonymous with "drone".  Forget that.  Why?

Any 'copter type aircraft is hideously fuel-inefficient compared to a fixed-wing because it's "suspending with thrust" instead of actually flying.  For many uses, that's a worthy trade-off, i.e. the ability to hover.  But the efficiency is bad - loitering surveillence aircraft would fly like gliders and probably look like gliders. Does the glider community have any rotary-wing gliders? No.
The quad/multi copter needs many engines and thus electric drive, which locks it out of using fuel for longer duration flight. A fixed-wing can go either way.  And R/C airplane sized engines are plentiful.
Payload capacity is much higher on a fixed-wing, so a meaningful warhead or sensor package could be carried.
Speed - which matters both for intercept, and a second thing.

The second thing is the ability to have a simple basic airframe with swappable mission modules. Some drones might carry a warhead. Others might carry a RADAR or other feature that needs a lot of electricity. Rather than complicate the basic drone design by adding a generator to some engines, do as the military does and build the module with a generator called a RAT (Ram Air Turbine). Instead of belt drive, airflow drive. Less to go wrong.  But the RAT requires airspeed beyond what is possible for a quadcopter, but is easy for a fixed-wing.  In fact, I would even prefer a RAT for the basic electrical needs of the drone. A RAT is just a generator attached to a prop, in fact every quadcopter comes with 4 of them lol.  BLDC motors work as generators.
And they could fly high but they don't have to because the above measures have pressured the enemy into the narrow zone below 400' AGL.  In fact, having the interceptors loiter high would give them the ability to dive to gain energy/speed.
Nothing against a quadcopter - they're a good fit for the task size and attention span of hobby and task-focused drone flyers.  But a sustained air surveillance or combat air patrol needs much more endurance than that, so for your task I'd absolutely say fixed-wing is the way to go.
Now, network them.
The general idea I see is to have some drones tasked with surveillance and others tasked with intercept.  Surveillance could be as easy as listening - using the sound of aircraft to locate them - the drones have accurate position sensing and accurate clocks, and report contacts to each other via radio. That makes it easy to triangulate targets.  Digital signal processing of that sound could identify right down to model of aircraft.  You can't outrun Motorola, so the intercept drones are getting a picture of the track of the enemy even while it's a long way off. They have time to position to "get in its way".
There would be technical tricks to resolve regarding sensors, geolocation (since the enemy can be counted on to mess with GPS/GLONASS), and telecommunication - you might actually have to read Hedy Lamarr's work on spread-spectrum... not using a single frequency subject to jamming or RDF. See also Qualcomm's work with CDMA cellular, or Metricom's lovely-for-the-time "Ricochet" - a wireless mesh network that did not need the backbone -- Ricochet modems could talk peer-peer.  It might be as simple as 25-year-old Ricochet modems in very drone.
Another thing this network could do is dull the senses of pilots to MANPAD launches by having $20 decoy rockets all over the place designed to mimic a MANPAD launch. They would listen to the network and fake out just as the aircraft is overhead.  At worst the pilots use up their flares on decoys so they're Winchester when they meet a real one. At best the pilots treat everything as a decoy and are blindsided by the real McCoy.
Once the hardware is in place, the performance of the system is mostly down to software.
So yes, possible. Very possible.  Because you've reduced the battlespace to 2-D, or really a line of a particular width.

What looks like a simple task at first, becomes quite impossible, when you start comparing the exposed area of let's say a fighter-jet or a helicopter to the humongous volume of airspace around it. You would have to know exactly where to place your swarm in order to effectively intercept the hostile aircraft.
I assume that you are referring to a mine-field like design. In that case, the same problems apply to your drones, that apply to ground based mine fields: You have to know where to place them. Just with the difference of having to deal with roads (less possible area) instead of airspace (a lot of volume).
Assuming you have soooo insanely many drones, that you could actually reliably hit hostine aircraft, you would have to equip every drone with a proximity fused IED or other explosive payload, which is heavy. It's not like you could take down an aircraft with a firecracker.
Assuming a drone might hit an engine is not worth the thought. As I mentioned before, even the entire surface area of a military aircraft is insanely hard to hit - concidering the placement issue mentioned -, not to mention hitting a single engine intake (disregarding the fact that many russian military aircraft like for example the SU-27 or the KA-60 have twin-engine architectures and can deal fairly well with a single-engine-failure).
Laser-pointers... don't do any damage. They say, a laser-pointer might blind the pilot for a second, but even in civil aviation this is merely a hypothetical problem. And here, the "hitting-problem" also gets to another scale: Now you are trying to hit an aircraft with a 2mm-dia beam. "Improbable as hell" is an understatement.
And gallium, same problem. Hitting is nearly impossible. And the potential damage is minimal. It might work well under lab conditions with a lot of galium on an exposed aircraft wing, but not in action on a battlefield. The airframe of an aircraft is protected by an oxide-layer and paint. No way a very slow reaction could ever harm the aircraft (if it even stays on the surface while the craft is in motion)
All in all, one multi-ten-thousand "cash" AA-missile is way more effective at reliably taking an aircraft down, than hundreds of drones over a city (if someone could even create such a swarm in a realistic period of time, or even pay something like that. One or two drones are cheap. Hundreds of them to hit one craft... SAMs are more economic!)

Because the usual off the shelf drones have extremely bad loiter times and you could not rely on chance to hit an airplane but would need fancy targetting algorithms (and probably feed the algorithm with data from ground based radar).
With off the shelf drones, usually you get quad copters. They have short battery lifes. And without a lot of custom software you can be absolutely sure that there is no better way to hit an aircraft than chance - maybe arranging them in curtains intersecting possible flight routes, but thats still square kilometers with at least one drone per square meter to get any useful effect.
I think it's quite a plausible idea, with a purpose-built swarm of military drones (sure as hell not n-copters, more likely small planes with the capability to ride thermals autonomously) designed to be sucked into jet engines. But then you would need to be able to anticipate an aircraft before it arrives, drawing the swarm in close around it's flight path. Most likely this means you need radar coverage of the airspace, some system to mark detected planes as targets and then send the drones a kill order. And the drones would need to have some autonomy, because it's easy to jam a signal such that low power drones have no chance to receive or send data. And the drones would need some way to head for the engine and get sucked in, because that's where you can do a lot of damage with little mass, that might imply machine learning algorithms (because the engine intake can probably not be targetted with a simple heat seeker).
So... cutting edge military technology with serious moral implications especially before there is enough experience with it to make it "safe". No way to do it with off the shelf drones.

Consumer drones are a danger to aircraft that are landing or taking off.
It may be possible to shut down an enemy airport with a swarm of drones, if it's being used to ferry in supplies and personnel. It would be bad for morale to use such an airport without clearing the drones first. Their low ceiling prevents using them higher up.
Combat aircraft can choose their flight altitude to suit the conditions and the threat profile. In general, flying low is done for two purposes:

Get obscured by terrain to penetrate long-range air defenses.
Deliver precision strikes that achieve objectives with fewer flights and less collateral damage.

In asymmetric warfare, low altitudes are already dangerous, since they can be reached by low-tech AAA and man-portable weapons such as MANPADS. These are greater dangers than a risk of collision with a drone.
If necessary, the opponent can choose to fight from higher altitudes. This will decrease the precision of non-guided munitions and require more flights to achieve the same objectives. Guided munitions can be used from altitude.
Rotorcraft don't have that choice. They almost always fly at low altitude, due to how they get their lift and to their operational role. However, combat helicopters have their intakes protected, and most of them are armored.
They don't fly fast enough for a small drone collision to be very dangerous on its own, and small drones don't carry enough payload to effectively deal with the armor. MANPADS, on the other hand, and sometimes even RPGs, are quite effective against low-velocity, low-altitude targets.
In conclusion: technically possible, but the only viable targets are airports that aren't well-patrolled on the ground.

The sky is huge with regards to volume.  And planes are fast.  I imagine you would need thousands upon thousands of them to be put in front of a path of a fast moving aircraft so that the plane can not just easily go around them.
But for a second, lets imagine a world where this is possible.  Drones would need to be:

fast... able to arrive to the ever changing path of the aircraft.
have a long lasting battery that would let it arrive from its base to the calculated trajectory.  This might be several miles.
stupid cheap to be able to have the thousands of them available.
maybe not as important if it can get into the right parts of the plane... able to carry munitions.

I suspect a better way to arrive to this might be from starting at loitering munitions.  Loitering munitions can typically remain in the air patrolling.  Then when they go into action, they turn into a missile of sorts.  So this is a mixture of drone tech (for the loitering part), then missile tech for the final part.  Maybe over time this can evolve into something that starts becoming more drone and less missile over time?

As with many other novel tactics developed in the past, battleship lovers are having a difficult time accepting "the times, they are a changing".  Reference Ukraine 2022 for the results.  Cheap drones will be very effective in the future as an integrated part of air/land defense.

+1 but it's Worth noting that the US variants of these (switchblade, coyote) have operational ranges and times that are fairly small -- an hour at most.  They are also much slower than any aircraft so you'd additionally need to be extraordinarily lucky.  Really the problem is that drones are not a good tool for the task and even if you tried to square the circle there are already much better options out there.

@RobertDiGiovanni The Moskova died because it's operators were using their radar to watch a target too insignificant to be worth using a SAM.  It should have been able to tag those missiles but the only radar that would be expected to have found them has only a 180 degree field of view.  Maybe not too bad a design decision when the ship was built, but it leaves the ship horribly vulnerable to distraction by drones now.

Let us check the specs of an example drone, DJI Air 2s:

Top speed 68 km/h
Max climb/descent 6m/s
Range 18km
Weight 595g

That's pretty good already, but in order to damage a flying aircraft, the weapon has to catch up with it. These speeds, especially the climb speed, are so low that's not going to happen unless the target is a helicopter in stationary flight. Even this would require the helicopter to stay still long enough and close enough to where the drone is. Basically, it would require the drone to be already airborne, and the aircraft to fly almost straight into it.
In addition, if the drone flies into a helicopter... I wonder if it would be able to actually touch it. I mean, there's this big fan on top of the helicopter, blowing air at pretty high speeds, more than 100km/h straight down. Since this drone is specified to operate in winds up to 38 km/h only... I think the drone would just be blown away by the rotor downwash and never hit the helicopter. So it would have to attack from above and get sucked into the rotor blades, maybe try to damage them. But since it is tiny, would probably get through without being hit at all. Okay, maybe try to get sucked in by the tail rotor instead, while tumbling in the airflow from the main rotor and the turbine exhausts...
That doesn't bode well for success.
I think this type of drones will eventually be used, but for ground targets only, with a small explosive charge. It should be possible to use computer vision to target a moving tank's sensors, or get it to fly into a building and blow up someone important, for example. This would need quite smart software, because both GPS and the radio link from the drone to its operator will be jammed, and a drone is too small and cheap for accurate inertial guidance.
Besides that, as someone said, "When you merely wish to bury bombs, there is no limit to the size." Therefore, IEDs seem a much more viable option for a low-tech military.

If you anticipate where your target will be flying, there is no "catching up with" it involved. Simply put the swarm in its flightpath & let it come to you.

For the helicopter, you could have two drones with a rope between them. Assuming the rope is stable enough, if they get sucked through the fan they could damage it.

I'll take "drones" in this case to mean quadcopters. Consumer quadcopters don't have very long flight times- 20 or 30 minutes maximum. The battery charge time for the type of battery common to toy- and hobby-grade quadcopters is often an hour or more. It's just not feasible to sustain a fleet large enough to be considered for an anti-aircraft swarm. Further, unless you plan on making each aircraft autonomous (consuming time, money, manpower, and supplies), each aircraft will have to be controlled via radio frequency, which is susceptible to jamming. Finally, as others have noted, the kill probability is extremely low relative to other weapons systems, giving a low yield for a highly maintenance-intensive system.

Generally speaking, civilian radio equipment is not up to the task of operating in a battlefield electromagnetic environment. In the U.S., at least, the FCC makes quite clear that civilian devices need to be specifically designed to be overwhelmed by law enforcement/military devices ("accept all interference" is the specific terminology I recall).  Once current generation aircraft are in the area, you're extremely likely to lose contact with the swarm unless you have non-radio means of directing their activity.  Pre-programmed deployment patterns, or just brute-forcing and trying to fly the swarm visually from the ground (your ground station is likely to be robust enough to break through jamming), suffer from the costs associated with complex pre-development or a general lack of acuity.
In short, these types of systems are ambush predators, and highly unlikely to be effective against aircraft in flight at any reasonable speed, since they will struggle to meaningfully climb to intercept altitudes in the kind of time frame that you are alerted to enemy aircraft vectoring towards your launch site.
Trying to use them against hostile airfields runs afoul of operational range limitations of these devices - but would otherwise be your best option for weaponizing these systems. (Bird strikes are dangerous because they happen at low altitudes, and in otherwise vulnerable flight regimes.)
Remember that MANPADs are rocket-powered, and still have very short useful intercept ranges because of the relative speeds involved.
So between the extremely low odds of successful interception and the difficulties of controlling civilian equipment in the presence of radio jamming, it's just not likely to be a fruitful avenue for a military to use against a peer-opponent.

Wait a minute, you are asserting that in the US, there is a specific requirement that civilian drones be designed to be able to overwhelmed and overcome by interference from law enforcement or military devices?  I'm skeptical.  a) Are you really certain that this is what the phrase "accept all interference" means and b) are you sure that this specifically applies to systems for controlling unmanned aircraft systems.  I'm skeptical.  Maybe grounds for a new ASE question.

"Must accept all interference" doesn't mean "must be susceptible to interference". The specific legal definition is closer to "tolerate interference".

@HiddenWindshield The specific technical standard only specifies that the device cannot cause catastrophic harm as a result of receiving interference (it can't blow up or start fires, or if it would otherwise do so, it must have safeguards in place to prevent this, including operating environment restrictions).  Basically, if the fire department blocks the frequency by using a more powerful radio, the civilian-use device isn't allowed to up its own power until the fire department is drowned out, etc.  That's not exactly the meaning of 'tolerate' either.

@WilliamWalkerIII Actually, radios are allowed to increase their transmit power (up to whatever the limit for that particular radio or class of radios is) in response to interference. Your cell phone does this all the time. And it's not like a radio is somehow going to know the difference between interference from the fire department and any other type of interference.

@HiddenWindshield Well, my cell phone doesn't (or it's just terribly awful at it), since every time I walk past a traffic light, I completely lose my audio.  But the fact remains, there are limits to what is permissible and so practical constraints on how civilian equipment is engineered that essentially requires that such equipment be hardened or supplemented to carry out missions on a modern battlefield - modifications which buck the hypothetical the OP asks about.

@WilliamWalkerIII All consumer cell phones made after about 2005 (except maybe certain cheap knockoffs) will automatically adjust their power output in response to signal quality and strength information sent back from the cell tower. So unless your phone is one of these, then I'm pretty sure it's auto-adjusting its output power. The only legal limits on transmit power are there so that your device doesn't interfere with other people's devices, not because the law demands that your device be jammable.

And anyway, the "accept all interference" clause applies to the receiver, not the transmitter. If you know the encoding of the transmitter, there are techniques the receiver can use to still pick up the signal, despite jamming. All GPS receivers everywhere have to use this technique because of the low energy of GPS signals compared to random atmospheric noise, so it's readily available in the civilian world.

No, none of this is true.

There's some evidence that both sides are using civilian-grade tech quite effectively in Ukraine. But only against ground targets, such as using the drones as artillery spotters or dropping light munitions from them.


text:
Exactly when do the ARMED Speed Brakes extend after landing a B737-800?
When do the armed speed brakes of a B737-800 extend after landing?
Is it when the main gear makes contact with the ground, or must the nose gear also be on the ground?

The system will extend the flight spoilers when any strut (either main gear or the nosewheel) compresses; the ground spoilers deploy after the right main gear strut compresses. Everything will deploy even if the nosewheel is held off the runway, although that's not a recommended landing technique.
Additionally, other conditions are required:

the system is armed (speed brake lever in the ARMED detent, and the SPEEDBRAKE ARMED light illuminated)
radio altitude below 10 feet
both throttles at idle

Additionally, the system will deploy the spoilers without detecting strut compression if it senses main gear wheels spinning up to 60+ knots (presumably both sides, but the manual I'm looking at isn't clear on that).
Source: an airline manual that isn't public domain, so no link or screenshot - sorry.

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text:
How does the turbulent boundary layer thickness affect separation?
I read that for a turbulent boundary layer, a thicker (turbulent) boundary layer separates earlier than a thinner (turbulent) boundary layer? Why is that? Or is it the other way around?

On a flat plate the thickness of the boundary layer grows along the flow path without a tendency to separate. Therefore, thickness alone is not the culprit.
However, when the pressure gradient in flow direction becomes positive (increasing pressure), the flow will slow down and thickness grows much faster than in non-decelerating flow. Now separation happens when the speed at the surface drops to zero. Right before separation you will see an exponential growth of the boundary layer, but it is only an indication for the decelerating flow which really is to blame for the separation.

Turbulent boundary layer is thicker than laminar boundary layer as rightly pointed out, for any given distance travelled by fluid over a surface. However, thickness is not directly related to separation. The separation results when the fluid doesn't have enough energy (read momentum) to counter an adverse pressure gradient (typically a divergent section).
Turbulent flow has more mixing between the layers and can survive more adverse pressure gradient than a laminar flow (which has no mixing between the layers). So for the same surface geometry, a turbulent boundary layer would separate later (or may not even separate) compared to a laminar boundary layer. The same idea is used in turbulators or vortex generators to keep the flow attached to the surface.

I understand that a turbulent boundary layer would separate later compared to a laminar boundary layer. But given a turbulent boundary layer, would a thicker one separate earlier than a thinner one (as asked in the original question)? And why?

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text:
Is there an example of a classic aviation engineering moment when engineers had to discard all their work due to the wrong approach?
Is there any classic aviation/space engineering moment where the engineers had been working on a concept for months and then realised that their approach to the problem wasn't quite right and so had to discard all their hard work and re-start from scratch? Please could you give some examples?

Langley, except he never got around to the re-start https://en.wikipedia.org/wiki/Samuel_Langley

There was a lot of that in the early years of aviation, pre-1900's.

@RonBeyer Please could you give some examples?

Do you count square windows on comet as an example?

@vasin1987 that wasn't a discard and start over thing.  It was simply stress calcs that were off for the corner radius used.  If thicker material/more rivets were used, it wouldn't have been a problem.

Well the DeHavilland Dash 8 was basically a do-over of the Dash 7, designed for a marketing concept that never materialized.  Only about 103 -7s were built, sold to operators that really needed its special capabilities.

Nevermind months - how about four decades?  They still haven't gotten to the point where they realize it's a doomed idea...

@vasin1987 The Comet windows were not discarded, never mind the Comet itself. The Mk.4 had rounded corners to its windows and additional strengthening, it was a perfectly good aeroplane. (Its only problem was that while everybody was waiting, the Boeing 707 arrived).

@J... for a moment I thought "the F-35 isn't quite THAT old, is it?"

I'm not sure if this meets your criteria, but would the Concorde/Tupolev (and later proposed-but-not-built supersonic passenger planes) fit the bill?  Might not count since some of these actually were in use for a while before they decided it was a bad idea and scrapped the programs.

The Space Shuttle program could partially match your criteria. It was discarded after the Challenger and Columbia accidents, although it somehow worked, although with a pretty high accident rate, even for space exploration.

@vasin1987: The Comet 1's windows actually weren't much squarer than airliner windows today - the actual problem was that the builders used punch riveting (which is prolifically microcrack-producing) to attach the window frames, when the manufacturer's plans specified that they were to be glued in place (without using any rivets at all).

The first failed attempt by Rolls-Royce at using composite materials for jet engine fan blades, in the original RB211 engine design for the Lockheed L1011 TriStar, not only led to "discarding all the work" but bankruptcy and nationalization of the company in 1971.
There was no way that the contracts with Lockheed could be made profitable after changing to a conventional (metal) blade design, and the bankruptcy forced the cancellation of the original contract and re-negotiation of a new one.
The long term outcome was that the RB211 family of engine designs is still going strong (and is profitable) 50 years later - and the latest members of the family now have composite fan blades.

Yeah, and that mess took the Tristar with it.  Great airplane.  Talk about work for nothing.

It was a great plane. At least it lives on as a gateway to space.

Actually, the earlier Conway engine also had composite fan blades, at least in later versions.  That was entirely successful and used for many years in the VC-10 airliner.  It was when scaling the blade up for the RB211 that problems appeared.

Filing bankcruptcy in this case is a black belt business move.

@lalala and hey presto, they were nationalized by a Conservative government!

Supermarine Type 300
There are plenty of examples, but if you want a classic then perhaps the Supermarine Type 300 fits the bill. It began as a cleaned-up Type 224 but, as first presented to the Air Ministry, retained the Rolls-Royce engine with evaporative steam cooling and a straight, tapered wing with room for around four machine guns.
While it was still on the drawing-board the PV12 engine moved to glycol cooling and "Merlin" monicker, the Ministry requirement to eight guns and the wing to thinner profile and elliptical planform. Yeah, the Spitfire.
That famous D-section main wing box is in fact the legacy of the evaporative cooling system.
De Havilland DH 106
The world's first jetliner is pretty classic too. When de Havilland started work on a revolutionary idea of a jet-powered airliner, they turned to the German secrets being brought back from the aftermath of war. The airliner was to have a tailless swept wing. They built a quarter-scale aerodynamic test plane, the DH 108. It was fast all right, but it came at a time when nobody understood the sound barrier and all three examples shook to bits, killing their pilots. In part that gave the shape an undeserved bad name, but mainly DH realised that it did not have sufficient trim tolerance of varying CG position during commercial operations, and a tail would fix that. Enter the DH 106 Comet, the world's first commercial jetliner.
Convair F-102/106
Here's another almost as classic. When Convair first flew the prototype delta-winged jet interceptor the YF-102, it sucked. It handled like a pig on the approach and couldn't even go supersonic in level flight. Two new discoveries at that moment included area-ruling and conical leading-edge camber. Both were applied in somewhat ad hoc and hurried fashion to the YF-102A and it made Mach 1.25 in level flight, while also being a lot tamer on the landing flight path. Time for a breather and a proper job with 50% more thrust to boot, and the F-102B, renamed as the F-106A, happily hit M 2.3.
Fairey Rotodyne
And one more for luck. In the 1950s Fairey Aviation developed the Rotodyne, a VTOL feederliner with conventional wing and propellers but also a tipjet-driven rotor. Complicated and expensive yes, but what ended the project was the noise made by the prototype - its tipjets sounded like a thousand banshees coming in for the kill.

The DH Comet also had the benefit of square cornered windows which, unfortunately, led to stress concentration and hence, fuselage failure mid-flight. It led to a complete rethink of window shapes and the now ubiquitous rounded windows.

@FreeMan Rounding off the corners of the openings is hardly discarding all one's work. Far greater changes get made all the time. Many earlier aircraft had had rounded windows, they were nothing new. What changed was the increasing popularity of stressed-skin construction, where reducing stress concentrations matters more.

The original design for the 747 had a full double deck all the way down the fuselage (similar to the A380), but discovered that evacuating from the upper deck would have been difficult and dangerous. That design was scrapped, paving the way for the widebody 747 we know today.
Source: Joe Sutter, 747 chief design engineer

From the same book, the 314 Clipper used to have a traditional single tail. It went through the double outside plates to the final three during its test flights.

The Republic XF-84H "Thunderscreech" was an attempt to improve power responsiveness by using a turboprop to replace the turbojet in a transsonic upgrade to the subsonic F-84.
The supersonic tips of the propeller, however, made so much noise that ground crews became physically ill.  There were other problems, as well -- engine reliability issues and aerodynamic problems.  The airplane flew, but it never flew well, never set the propeller-powered speed records that had been expected of it, and never went into even limited production -- and further, supersonic tip propellers were abandoned industry-wide for three decades or so until "prop-fan" designs with highly swept blades were proposed and tested (but never adopted) for transport aircraft.

If you're going to count X-planes then this will become a long list...

@leftaroundabout Lots of X-plane concepts were "what happens if" experiments.  Republic was pretty sure they could break Mach 1 on a propeller, and solve the slow throttle response of direct thrust turbines (by being able to almost instantly change pitch).  Whole concept out the window before first takeoff...

If I remember correctly, the TU-95 "Bear" patrol bomber of the Russians has supersonic propeller tips.

@CalinCeteras I've read that, too, though I think they're barely supersonic, and the bomber itself is firmly subsonic, as compared to the XF-84H which was high-subsonic and intended to fly supersonic, with significantly supersonic tips.  And the Bear is pretty danged loud, too...

Another example would be large passenger airships.  Lighter than air or heated gasses were the only means of providing enough vertical lift for reliable human flight until the first powered winged aircraft struggled into the air at the turn of the 20th century.
Airships benefitted from these new engines too, developing into skyscraper sized trans oceanic giants.
However as further improvements in propulsion made ever greater thrust available, the drag equation shifted more towards forward speed as well as lifting requirements (around 100 knots and beyond) in the 1930s. The large airships, with their draggy gasbags, became obsolete and were progressively replaced with flying boats, piston airliners, and jet airliners.
Issues with weather, similar to the Ever Given in the Suez canal (huge surface area becoming uncontrollable in high winds), also contributed to the end of their days.

Of course, the idea of passenger airships isn't quite dead.

This one might take up a bit less hanger space.

And this one might have done well with turboprops.

On a related note, the Hindenberg may qualify as a "oh bother... scrap it all and start from scratch" moment!

The famous B-52
When Boeing engineers designed the B-52, it was to have straight wings and propeller engines, just like its predecessors. When on one faithful Friday the time came for Chief engineer Ed Wells and and two of his fellow senior engineers to present the design to the Air Force, the USAF rejected the design and demanded a swept wing and jet engines instead. Ed and his team returned to Ed's hotel room, but not before having bought a block of balsa wood. Over the weekend they then proceeded to redesign the B-52 with swept wings and jet engines, inside Ed's hotel room, preparing a full report and a balsa scale model. They then returned the following Monday to the Air Force brass who, impressed by the effort, approved an initial order of 13 B-52 bombers.

That's wild. "We've spent months creating this design using proven techniques and carefully made the calculations to meet every one of your requirements." "No, that's no good." "Ok, then how about this model we slapped together over the weekend? Who knows if it works, but it looks really cool!" "Shut up and take our money!"

This is curious because usually it's the opposite: the military insist on a robust proven/working design (and quick!) instead of the new fancy risky stuff. Still, this was at the preliminary design stage, when such things happen routinely. Unlike B-52, several T-10/Su-27 were already built and flying when the design was scrapped and started anew.

Depending on your criteria, this might qualify:
Airbus scrapped a concept, and redesigned it to make the A350. The original idea was to use A330 fuselage sections to create a competitor to Boeing 787, but after a number of bigger clients expressed their dissatisfaction  with the original approach, Airbus had no other choice than going back to the drawing board.
This may actually have been more of a management level mistake, rather than just the engineers getting sidetracked, but I bet engineers had their say when choosing the ill fated "lets whip up another 330" path was at hand.

Another Airbus example would be the wing of the A310 which was part of the British work share. Then Airbus Bremen presented their design and BAe had to retract theirs and, in order to keep face, the official story goes that they designed a new one which eerily resembled the Bremen design.

In commercial aviation it's definitely the BD-5.    Jim Bede sold thousands of kits for this small sport plane, before he had a working engine.  There is a lot of history recorded about trying various small engines that would fail after a few hours.  Meanwhile he was selling more kits to fund his day to day operations, and eventually went bankrupt, leaving all the kit owners hanging.  Incomplete kits with no engine.  One to my dad in the mid 70's.
Some were completed, but the bulk of kits sold and contracted for never saw the light of day. Bede didn't now how to say no to a customer's money, even though he never had a viable engine.
https://en.wikipedia.org/wiki/Bede_BD-5

I would have to disagree. The early BD-5 design with the V-tail and composite construction was quickly discarded, but the redesign was successful and many thousands of kits were delivered, albeit without the engine, for reasons unrelated to the aircraft design. This is all widely documented.

Sukhoi Su-27
The prototype T-10 was designed in the early 1970s as a direct response to F-15 (which was being finalised then). By the mid-70s it turned out that T-10 was underperforming and would lose to F-15 overall.
Mikhail Simonov, its chief designer, insisted on a new design (against a substantial government pressure). The new design, coded T-10C and which would become Su-27, had amost nothing to do with T-10 except for general principles: integral aerodynamic design, fly-by-wire with reduced/negative static stability, etc. As Simonov put it, only the main wheels and the ejection seat remained from T-10.
As a result, Su-27 will probably forever remain a pinnacle of aerodynamic design for agile piloted fighters.

A couple more candidates : the Avro Manchester deserves a mention. The (arguable) mistake was to make a heavy bomber as a twin engine aircraft, using bleeding edge 24 cylinder engines without the luxury of time to work out their reliability problems.
A relatively simple re-design with 4 reliable Merlin engines, readily available in mass production, gave us the Lancaster (saying anything more would be redundant). As such, the airframe engineers didn't lose much of their work, though the engine design didn't go any further.
Another candidate might not deserve to be on the list : the BAC TSR-2 of which Sir Sydney Camm said : "All modern aircraft have four dimensions: span, length, height and politics. TSR-2 simply got the first three right."
I would not agree that Concorde should be on the list because it entered production and served for decades; further sales simply didn't materialise. However you might consider the Boeing SST which started out as variable geometry, was re-designed as a Concorde-like delta wing, and then cancelled when two things became clear : like the TSR-2 and Concorde, it got the politics wrong, and the 747 was about to be a big winner.

Yes, it was Shackleton who axed the TSR2, but this was a Mach-2 Nuclear bomber: to drop bombs on...?  Same could have happened to Dyna Soar, able to cary a nuclear weapon, go down from orbit, re-start engine, and switch to a fully diffdrent orbit, a fully diferent destination. Simply, weapon of too much supremacy. Blessings +

When Theodore von Kármán was visiting, along with US Army officers, the German seized Aerodynamic research sites, German engineers were deported to the US, USSR, France,...he made an urgent call to his company headquarters in USA, indicating to discontinue all designs made for what later became the B-47, after watching results of sweptback wing, a concept spread in a High Speed Aerodynamics congress held in Italy in 1937. Blessings +

The one I know is the B-49 series .... to the XB 35 flying wing :
As far as I know were the control systems outdated and the machine became
an elephant to steer , in critical situations . Though many , many
improvements and with the ONLY technical systems they had , that time , even when it did reasonably well in the end :
EACH AND EVERY ONE of the XB 35 flying wing got scrapped , literally .
They built from their experience on , on the B2 spirit , though , so scrapping
a plan does not mean that the experience , made upon it , is also lost .
By the way , that happens all the time : from steam engines to diesel to
electric , ... nobody wants to heat up a locomotive in the morning for 5! hours anymore ... , just to start it up .
The final decision for the XB 35 flying wing seems the weight and complexity of the control
systems : nowadays make electronics the things much lighter and those
airplanes were designed for tomorrow but had the technology of the 40's .
Think about it for a moment : even the Liberty ships , that freed Europe
at the end of WW2 had TRIPLE EXPANSION STEAM engines . Diesel ?  - Existed ,
Double - Action Steam Engines ? - Existed .... so what I mean : sometimes
they are bold enough to do the opposite : taking a step back for several good reasons .
Here 's a video on the development of the B 49 / YB 49 and XB35 flying wing

LATER EDIT : some " numbers " for this extraordinary bomber , taken out of the book
" VLIEGTUIGEN " by Enzo Agelucci :
first , the text about the NORTHROP XB-35 FLYING WING ( 1947 , USA )
translated from dutch into english :
On the 25th of june , 1946 , the first flight was made with the " flying wing " , as Northrop calls this type of airplane without fuselage or tail .
That was the N9M , which was just a third in size , compared to the later XB-35 .
The test flights were satisfying and 3 prototypes of the bigger B-35 bomber got built . Already in 1941 , they began with the design .
As a result of technical difficulties , the XB-35 , which could carry
20 machine guns and a capacity of 41200 pounds of bombs , was not built in series production . Two of the prototypes got equiped later on , with jet engines but also these tests were not successfully .
Technical data ( the " green " pages out of the end of the book )
name : Northrop XB-35 Flying Wing .
built by : Northrop Corp. , USA .
type : bomber .
year of introduction : 1947 or later .
engines : 4 x Pratt and Whittney R-4360 jet turbines , each 3000 SAE HP .
wingspan : 172 Ft or 52.43 meters .
length : 53 Ft or 16.15 meters .
high : 20.0 Ft or 6.10meters .
empty mass : 89560 pounds or 40.624 Kgs .
maximum takeoff - mass : 209000 pounds or 94802 Kgs .
maximum speed : 391 miles/h or 629 kms/h .
cruising speed : 183 miles/h or 294 kms/h .
range : 10000 miles or 16100 kms .
crew : 9 brave guys .
The rather small cruising speed confirms : it steered like an elephant , maybe they got redundancy in some kind of way but
with all the heavy equipment and updates that added to this , the design specs were NOT met .
To be honest , if you drop 40000 ponds of bombs , you GOTTA get away FAST ;-)

There were only two XB-35s made; "each and every" is a tad misleading. You are probably thinking of the YB-35; around half a dozen were completed and several were then converted to jet power as the YB-49/YRB-49A. "Scrapped" is also an understatement; "Wilfully destroyed" is closer to the mark - the Smithsonian were even refused an example! Also, the B-2 can hardly be seen as a retread, stealth and fly-by-wire made it hole new ballgame.

Dear Guy , That is probably the best reason they got scrapped : even when you want to steer the airplane with hydraulics , is it a simple pump and electric valves that control the motion . The steering of it becomes totally electronic but back then they fed hydraulic lines all over the airplane . Hardly any redundancy over the controls was normal back then .

Even if you regard hydraulics as obsolescent by then (which is highly debatable), that does not make it "wrong". They "had the technology of the 40s" because they were designed and built in the 40s! That was right for their time, not wrong.

Dear Guy , I know that hydraulics can also be redundant but the fact is that all this mass must be taken into account . I also understand if they had 4-bit comparators that time , reliable enough , they would have introduced fly-by-wire much sooner . But THIS is the " design failure " category of the website . And the XB - 35 fits right into it . Yes , I have deeply respect for those engineers who built this massive airplane in a still much bigger challenge to be the first airplane of this kind  , no doubt . Being the first also means to BE the first to take punches for ( little ) mistakes .

These comments are for discussing your answer, not for repeating it. So far nobody is agreeing with you. Leave it at that.

A-380 is an example of wrong desing, its wingtip vortices, wake turbulence being so severe, it forces airports to stop traffic for several minutes, allowing for safe use of space by smaller airplanes, a Gulfstream sized Jet that entered the A-380 wake turbulence was so severely structurally damaged, that needed being scrapped. A-380, Antonov An-225 are the extreme class of wake turbulence of its own. But it was market, not engineers,who sent down this machine. Blessings +


text:
Examples for the benefits and importance of having a Boundary Layer
I understand what a boundary layer is and how it is formed. Mostly these boundary layers are related to increasing skin friction drag of the body. Please could you give some examples that show the positive effects of having a boundary layer i.e why boundary layers might be essential and actually beneficial for certain applications? For instance, I read somewhere that airplanes wouldn't be able to fly without a boundary layer.

Since boundary layers are impossible to avoid, only theoretical considerations without practical proof are the base of the claim that a boundary layer is needed for flight.
The general idea goes like this: In purely potential flow lift is only possible when the rear stagnation point is artificially fixed at the trailing edge. Without the loss of energy in the boundary layer, the flow around an airfoil approaching the trailing edge would just negotiate it and combine with the flow around the opposite side at a point where no circulation results. Only the slowing at the surface by friction will make this flow around the trailing edge impossible and force the rear stagnation point to the trailing edge.
I once witnessed a presentation in which the researcher tried to show that even in potential flow the sharp trailing edge will cause impossible flow conditions. In the end it looked to me like the scholasticism of the medieval monks: By changing the grid resolution, the flow parameters at the trailing edge changed, too. My conclusion is that the impossibility of lift in the absence of friction is impossible to prove conclusively.
Since friction saps energy, it is bad for efficiency. A more helpful example for a beneficial boundary layer might be temperature boundary layers which help to lower the wall temperature in a gas turbine below that of the core gas flow.


text:
What is this part of an aircraft (looks like a long thick pole sticking out of the back)?
On the website of Kenn Borek Air (an airline specializing in Arctic / Antarctic operations), on this page at the bottom, you can see the following picture:

It looks like a long thick pole sticking out of the aircraft (likely from the back?)
What is it?

Closely related, possibly a dupe

That is called a Survey Tail Boom and includes sensitive instruments that need to be mounted away from the aircraft body for clean measurements.
You can see the name of the manufacturer LCAS printed on the boom. In this case, it seems to be a tail boom for magnetic field measurements installed on a Douglas DC-3, which is used for Survey according to Borek Air's fleet information. The LCAS website has a detailed description of the tail boom:

LCAS’ fully certified (Supplemental Type Certificate) tail-mounted transition and boom assemblies carry the magnetometer sensor and requisite electronics. The aft part of the boom system contains the provisions for the magnetometer sensor, the mid section houses the amplifier and flux gate and the transition assembly can house optional electronics such as pressure/temp sensors. The transitions or forward boom assemblies can also be modified for installation of optional video cameras.  The orientation of the sensor is adjustable, to provide optimum coupling with the earth’s magnetic field. This innovation eliminates the traditional towed bird system and offers significant advantages in data quality, pilot work load reductions, survey flight efficiencies, and safety in all types of terrain.
The mod kit includes all the necessary doublers, support assemblies and attaching hardware, as well as new ‘end cap’ with Nav. light provisions, this allowing the aircraft to be flown without the tail boom and the OEM tail cone assembly to remain un-altered.

(lakecentral.com)

Also awesome for detecting submarines.

Are they using a DC-3 because taildraggers don't rotate on liftoff? How about landing with that thing?

@dotancohen The company also sells this survey boom for other aircraft with tricycle landing gear, but it looks like the boom is angled upwards on those aircraft. See e.g. the pictures on this page of the DHC-6 modification.


text:
Determining full route of domestic leg
I've come across a JetStar flight (JQ 28 on 3 February 2015) that's flying from Sydney to Melbourne which departs and arrives in international terminals.
My assumption is that it's part of an international flight - that is, it's flying from somewhere overseas to Sydney, and then going on to Melbourne, or that it's flying from Melbourne onwards to somewhere overseas.
I'm interested in whether it's coming from Japan. How can I find out what the full international route is?

it seems like you're trying to track someone in particular, and that's not what this site is about

I'm voting to close this question as off-topic because it is too narrow, and the answers will not help others who are looking for information here. Reword it in a general way, and it will be OK.

FlightAware.com is a good resource for this sort of thing. When you look up a flight, you can click "Track inbound flight" under the flight ID. If the trip on January 19th/20th is any indication, it arrived from Phuket, Thailand before continuing to Melbourne.

In addition to Flightaware, most airline websites have a published schedule so you can easily check the flights each week. However I can't seem to find it on the Jetstar site...

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text:
What is a 'Wing Loop Fault' on an A320 series Aircraft?
I was recently involved in a flight that had issues before take off. The plane was an Airbus 321.
It was explained to me as being a "wing loop fault". The problem was that the ECAM messages were displayed differently each time and were inconsistent. The flight was aborted just before take off (for safety reasons) but the problem was fixed soon after by replacing a part.
Can anyone explain this problem with the wing loop to me in more detail. What could have gone wrong and what was the likely fix.
Also, what maintenance and pre-flight checks should occur on the "wing loop"?
Sorry for any vagueness in my question, I didn't receive much information.

Edits

This was on aircraft G-OMYJ around the 24th August 2013
My route was (EMA) --> (DLM), flight TCX5894. But the fault may have occurred on the previous flight. The details are unfortunately hazy. Is there anyway of me finding the whereabouts of the plane in the days leading up to this incident?

My guess is that there was some circuitry that was playing up. While I'm not sure about 'Wing Loops' specifically, for what I think is a similar example the Airbus A320 series has fire loops in the engines.
There are two fire loops. If I'm right, it's got little thermal sensors along way, which disrupt the current if they get too hot (or if the wire is broken). Logic gates ensure that both loops must be 'high' for the fire warning to go off. If one is not working, it works from a single input.
Assuming both of these were working from the beginning, I think it throws a FIRE DET FAULT warning on the pilot information display if one triggers without setting off the other one. I'd imagine that maintenance either disabled the faulty loop within those allowed by the Minimum Equipment List or by changing the part. If it was quick, it was probably the first, but they're not going to go telling passengers that they disabled safety equipment :)

Hi.. Can you provide the details of the "Minimum Equipment List" and which loops would be allowed to be disabled. Also, for the record, the resolution was that an unspecified part was replaced.

@Sam it's not quite easy to read off, but here's a copy: http://fsims.faa.gov/PICDetail.aspx?docId=M%20A-320%20R22a

Engine fire detection loops and wing overheat loops are different. The wing overheat loops are literally in the wings and run alongside the bleed ducts.

Wing loop fault refers to the overheat/leak detection system running along pneumatic lines in the wings and belly. Fire loops on the engines is a different matter. The loops consist of an outer and inner conductor separated by a "salt", when the loop gets warm, the salt becomes conductive and makes a circuit between the inner and outer conductor. The system often contains of two parallel loops, both has to detect a leak in order to give a warning to the pilot. If only one detects an overheat it gives a fault. The fault can also be triggered by a high resistance in the system, monitored by the BMC . Bleed Monitoring Computer. The system is build up by a lot of elements, and the connection between each element can be a bit "flimsy", making it difficult to perform a proper troubleshooting on.

This is the most accurate answer.

It might be a problem with the loops of sensors for the fire detection systems etc
For example: ECAM message "FIRE LOOP A FAULT" (ATA 49: APU)

Both engines and the APU each have two identical loops, A & B and a computer- FDU (Fire Detection Unit). A fire warning is given when both loops reach the proper overheat condition. If one loop fails the other loop is able to generate the warning by itself. A fire warning is given if both loops fail within 5 seconds of each other.
...
The leak detection system uses a single loop for the pylons and APU to detect hot air temps associated with duct leaks. Dual loops are used for the wings. If both of the dual loops detect a leak a warning is given, unless there is a fault on one, then only one loop is required to give a warning.

From Eric Parks training notes.

A loop fault is to do with the leak detection system for the pneumatics. As per the FCOM:

Leak detection loops detect any overheating near the hot air ducts in
the fuselage, pylons, and wings.
For the pylon and the wing, the sensing elements are tied to form a
double loop and for the APU, a single loop.
When the two wing/pylon loops detect a leak, or when one loop detects
the leak and the other one is inoperative, they activate a wing/pylon
leak signal.
BMC1 and BMC2 each contain identical control logic for the system.
A wing leak signal causes :

the bleed valve on the related side to close automatically

the associated FAULT light on the AIR COND panel to come on

the x-bleed valve to close automatically (except during an engine start)

the APU bleed valve to close automatically if the APU bleed valve is open and if the leak concerns the left wing (except during engine
start).

A pylon leak signal causes :

the bleed valve on the related side to close automatically

the FAULT light for the related engine on the AIR COND panel to come on

the x-bleed valve to close automatically (except during an engine start).

the APU bleed valve to close automatically if the APU bleed valve is open and if the leak concerns the pylon 1 (except during engine
start).

An APU leak signal causes :

the APU bleed valve to close automatically (except during engine start).

the FAULT light the APU BLEED pushbutton switch on the AIR COND panel to come on

the x-bleed valve to close automatically (except during an engine start).

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


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What is the purpose of the cabin temperature probe run from the gate?
I'm on a JetBlue flight and noticed a cable running from the gate / gangway into the plane, which appeared to be labeled "Cabin Temperature Probe" or something to that effect.  The cable is draped over the divider in front of the first row of seats. Further, the console outside the plane appeared to have only a single yellow light (which was off), no display or additional controls.
What is this for? It seems like a sensor actually installed in the plane would be more accurate (it would work even when the door is closed). As is, I imagine the probe is picking up much more ambient air than cabin air.


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Why are some contrails longer than others?
I've often noticed that planes flying on more or less the same track, headed in the same direction leaves different contrails. By different I mean how long they stay visible and the length of them. Why is this so? Is it weather related?

they depends on the weather conditions, if the air is dryer they are shorter to non-existant

Related: http://aviation.stackexchange.com/q/3089/95

Some areas of the country need more mind control drug than others.

It is a mixture of factors. All else being the same:

a contrail at a higher altitude will last longer (the lower temperature slows the sublimation of the ice cristals)
a contrail formed in air with higher humidity will be bigger and last longer (more water is being turned into ice)

If there is not enough (relative) humidity, the contrail will not form at all. See for example this image:

Image from NASA
The contrails start forming on the east, near the clouds where you have a higher humidity, and do not form on the west, where no cloud is present and you have higher temperature/lower humidity.

I've often noticed that planes flying on more or less the same track, headed in the same direction leaves different contrails.

What you observe might be airplanes flying at different altitudes and thus encountering different humidity/temperature combinations, giving birth to contrails with different histories.


text:
I don't quite understand the derivative in this vertical wind gust formula
First my english writing skill is still bad so pardon me if i accidentally made any mistake TT-TT.
So recently I have learnt about how the vertical gust affect the load factor and the calculation behind it, here are the formula in the slides:

So in the third picture I was wondering what dose the derivative of DCl/Da really mean and how these two derivative in the third picture equal.
It's frustrated how I dont understand that, please help me.

I don't understand TT minus TT ...

It seems to be saying the vertical gust causes a change in relative wind, which affects the Coefficient of Lift by changing the angle of attack in the same way as the elevator would in calm air.  So go on to tan alpha gust = u/V = change in AoA (therefor Clift) from gust.

is the V in the formula a TAS airspeed or IAS?

@CGCampbell   The statement TT-TT was in context with this person explaining they are learning English as a new language.  The abbreviation may mean Text Translation to Text Translation.  The book they are using as a text may not be in English...

The slides are indicating how lift coefficient $C_L$ changes with angle of attack $\alpha$ a few ways.
First, they conceptualize it as a discrete (finite) change in lift coefficient $\Delta C_{L_{gust}}$ due to a change in angle of attack $\Delta \alpha_{gust}$.
So, we can write something like this that is obviously true...
$\Delta C_{L_{gust}} = \frac{\Delta C_{L_{gust}}}{\Delta \alpha_{gust}}\, \Delta \alpha_{gust}$
Then, they want to generalize this from a particular change to any possible change.  They do this by recognizing that lift coefficient and angle of attack are linearly related and that this relation can introduced by the definition of the slope of the CL-alpha line.
$\Delta C_{L_{gust}} = \frac{d C_{L_{gust}}}{d \alpha_{gust}}\, \Delta \alpha_{gust}$
They then make the implicit assumption that the slope of the lift coefficient line due to a guest is the same as the normal slope of the lift coefficient line.
$\Delta C_{L_{gust}} = \frac{d C_{L}}{d \alpha}\, \Delta \alpha_{gust}$

Sorry for the competing answer! I started it last night but could not find the graphic I wanted to add. So I finished it just now in the already opened dialog (with I'm not a robot and all that) without noticing your answer. But they are different enough, so I let it stand.

@PeterKämpf no worries.  I have no problem with multiple takes on a subject.

The derivatives are the same because it doesn't make a difference what causes the change in angle of attack.
Normally, a plane will change its angle of attack when it maneuvers or changes speed. In a tight turn, for example, more lift is needed because the lift vector is tilted sideways (by rolling the plane) to effect the desired change in direction. Lift must be higher so the remaining vertical component of lift is sufficient to support the weight. When flying slowly, the attitude of the plane will be more nose-up in order to create the needed lift with the same wing.
What happens in a gust can be explained by simple vector addition. Normally, angle of attack is a consequence of attitude and movement. With a gust you need to add a wind speed, like that:

In still air, all what the wing "sees" is the wind speed resulting from its own movement. We usually call this ${\text v}_{\infty}$ because only at infinity, without influence from the wing's own pressure field, will this be the true wind speed.
Adding a gust is adding another wind vector. In my sketch this is an updraft (red, named w), so the direction is vertical and up. What the wing now "sees" is the effective wind speed ${\text v}_{\text{eff}}$, which comes at a different angle and so the wing experiences a change in angle of attack.
It does not matter in the big picture whether this change is from a change in pitch attitude or a change in wind angle. The gradient of lift change over that angle change will be the same.


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What difficulties may you find when controlling an aircraft with one engine on fire?
I curious that from a pilot perspective, what do they really feel when they experience an engine failure that caused by fire ? Can they easily maneuver the aircraft anymore, or will their situational awarenese decrease by time ?

Airplanes and helicopters are designed so that their control does not depend on the engines and they can be controlled even with no engine power.
The asymmetric thrust—in twin-engine plane with one engine stopped and the other running at full power—makes control harder and must be properly handled, which is why multi-engine pilots train it. The main problem is that if the plane gets too slow, the rudder won't be efficient enough to compensate for the asymmetry and the plane will roll out of control. This tends to be most difficult on light prop twins as they have the engines relatively further apart and have less excess thrust.
It does not really matter why the engine failed, the control problems will be similar. The problem with fire is that it can cause additional damage that can have additional adverse effects and it can get worse over time up to the aircraft falling apart if the fire is not extinguished. Often cutting off the fuel supply is enough to stop the fire since steel and aluminum do not burn, but if not, that's why there are extinguishing systems (at least on transport aircraft).
Fire is worse once the aircraft stops. While it's flying, the air stream is blowing the flames away from the structure limiting its spread, but when the plane stops, it can completely burn down in a couple of minutes. That's why there is the 90 seconds evacuation requirement. Still, there were some accidents when continued fire damage resulted in an aircraft eventually falling apart in flight (e.g. the infamous Concorde crash).
Situational awareness is a rather different thing. It means not whether the aircraft can be controlled, but rather whether the pilots know what they should and shouldn't do with it. Situational awareness is only going to improve as the pilots find out what the problem is.

@757toga  That is why their minimum speed is a lot lower than that of Part 25 certified designs. A lower minimum speed makes many more fields and roads into possible landing spots.

Like many metals, aluminum can burn if it is a powder.  If your aluminum airplane turns to powder, you were already in a lot of trouble.


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What proportion of a flight do planes spend in the upper atmosphere?
While creating a methodology for the carbon footprint of flights, I came across some information which required the calculation of the distance travelled by a flight in the upper atmosphere. Essentially, aircraft emissions at higher altitudes have a larger climate impact than ones at lower altitudes and to compensate for that, we multiply the emissions when the aircraft is in the upper atmosphere by a constant like 5.2.

It is recommended to apply the factor if possible only on the emissions in the higher atmosphere
because this allows for a better differentiation between short and long-distance flights

I have almost no knowledge about aviation and so it was hard for me to gather any information for how I could go about calculating the distance travelled in the upper atmosphere (and then calculate emissions based on the fuel consumption for that distance). I came across a diagram that showed the stages of flight and I guessed that I was attempting to find the proportion of the flight in cruise mode.

After going through this question on the aviation stack exchange, I figured a good estimate for the distance spent in the en route stage is the total distance travelled minus 60 NM, but I'm not sure if looking at the en route stage is what I want.

Could someone help me understand a way I could calculate the proportion of a flight that that the aircraft travels in the upper atmosphere? So far I'm expecting to multiply the total distance travelled by a given proportion, say 70%, given that the flight is long enough. Even if such a simple proportion isn't available, is there a way I could potentially calculate it given the origin and destination of a flight (and access to no financial resources for paid databases)?
Note: I was unaware that the 'upper atmosphere' had no standard meaning, so for this question defining it as above the troposphere is probably what I want as suggested by @Ben. This also restricts the question to commercial jets as pointed out by @jamesqf

I think I have to ask the obvious question, if you have almost no knowledge of aviation, are you the best person to be creating this methodology?  There are many secondary questions such as what do you mean by upper atmosphere, what pollutants are you interested in studying and so on.

How do you want to define "upper atmosphere"? Because aircraft tend to cruise near the boundary of the troposphere and the stratosphere, which are the two lowest levels of the atmosphere. No aircraft reaches the ionosphere/mesosphere which I would consider to be the upper atmosphere (but I'm sure some boffins have made their own definition for what counts as upper to suit their study goals)

A jet airliner cruising at something like 35,000’ does NOT start descending at 30nm. An old rule of thumb is 3 times your altitude, so 35,000’ x 3 = 105nm (3 nautical miles per 1000 feet )

@Ben: "aircraft tend to cruise near the boundary of the troposphere and the stratosphere"?  That's roughly 25K-50K feet (9-17 km).  Most of us probably don't often get above 14K ft or so (and the only times I've been higher have been in sailplanes).  Question needs to be limited to commercial jets & military aircraft.

@Frog I'm definitely not the best person to be creating this, but it's for a kind of extended school project and I wanted to explore the field as much as I could. I thought about the sort of ambiguous definition of 'upper atmosphere' but I didn't include that in the question to retain focus and thought that maybe the aviation industry has some way of defining it.

@Ben I think defining 'upper atmosphere' as the lower stratosphere might be what I need. The paper singled that out when talking about the multiplier.

@jamesqf I'll add the limitation to commercial jets in question, along with some of the other suggestions in this comment thread so far.

@DigitalDracula I definitely agree with the fact that a simple proportion like that wouldn't be a very accurate model. If we consider an 'optimal' flight (it travels in a straight line, a simple ascent, cruise, descent, etc.) by a commercial aircraft, do you think approximating the cruise height of the aircraft (maybe making this aircraft-specific), finding the distance required for the ascent & descent (finding aircraft-specific ascent & descent rates) and thus the cruise distance might work to some extent?

do you think this is an approach worth trying? or is there too much variability in which a model like this might not even work for an average case? In which case, am I better off just not considering the multiplier factor for just the stratosphere?

@DigitalDracula In either case, thanks for your help, it really was quite useful :) I suppose I'll move on to other tasks regarding the calculator then.

I would challenge the multiplier.  Aircraft create vastly more emissions during the climb than during cruise.

The aviation part is relatively easy. But you have to justify the effects you ascribe to the carbon dioxide released at different levels, and that in turn requires diving deep into physics of atmosphere.

@MichaelHall Here's the source for the multiplier if you're interested: https://www.researchgate.net/publication/327551417_Aviation_and_Climate_Change_Best_practice_for_calculation_of_the_global_warming_potential. I believe that this multiplier has to do with the chemical and physical reactions that take place at that altitude and less with the amount of emissions. The multiplier is multiplied with the emissions at that level (which may be lower than the ascent).

The total emissions are calculated using the fuel consumption database, given by the ICAO in Appendix C: https://www.icao.int/environmental-protection/CarbonOffset/Documents/Methodology%20ICAO%20Carbon%20Calculator_v11-2018.pdf. I believe this adjusts for the larger emissions during ascent since the slope of fuel consumption for lower distance flights is larger than ones for longer flights.

The question is clear: do we on the Aviation site know what is defined as the Upper Atmosphere, referenced in an article on pollution. Voted to keep open.

From an unverified source, a typical rate of climb is 1800ft/min and the stratosphere is reckoned to begin at 33000ft.  If we assume that the flight begins at sea level and that the descent is also at 1800ft/min then the aircraft will be below the stratosphere for 36 minutes and 40 seconds, and the remainder of the flight will be in the stratosphere.  You would then need to estimate the average duration of a flight, and consider short hops where the maximum altitude may be lower.  Perhaps look at flightradar24.com, grab a snapshot of what’s happening at one instant and assume that it’s representative of typical conditions.  If there’s too much data to look at then select a random subset and analyse them.  There are several assumptions here, so be sure to consider how valid they are and adjust your methods accordingly.

International Standard Atmosphere has tropopause at 11 km, that is 36,000 ft, not 33,000. But actual height of tropopause varies wildly—from 9 km (30,000 ft) to 17 km (56,000 ft). Since airliners fly mostly between 32,000 ft and 40,000 ft, this variation cannot be neglected.

This may work since we also have a database of the altitudes of all airports, it could help make our calculations more accurate. Do you know how we could calculate the ascent and descent in distance (ie using the groundspeed during the climb) rather than time? We could then subtract that distance from the total flight distance rather than estimating time.

@JanHudec although that's a pretty valid concern, I don't think it's worth investing too much into or trying to take it into account because the number of variables involved are too many. Unfortunately, I think just assuming that the 'upper atmosphere' is the cruise height of the aircraft (whatever that may be given that it's around 33/36,000 feet), might be the most we can do.

@BhavyeMathur if that's most you think you can do, then you can just suck some numbers out of your finger and it will be just as good. But if you want your results to have some actual relevance, you absolutely have to understand why you give carbon dioxide released at different altitudes the different signifcance you do.


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Will a stalled aircraft fall down like a brick? Or how steep does the Lift-AoA curve fall off after stall?
When an aircraft stalls, do all or most lift forces abruptly disappear or is this transition continuous?
Will the aircraft drop from the sky like a brick or can this (losing lift and altitude) happen so gradually that it will be entirely unnoticed from the crew and passengers?
I'm troubled by this sentence on Wikipedia referring to the AF447 disaster:
"The aircraft remained stalled during its entire 3 minute 30 second descent from 38,000 feet (12,000 m)"
(source: https://en.wikipedia.org/wiki/Air_France_Flight_447#Accident)
It seems the crew were unaware of the stall condition. (But let's not discuss the reasons here)
If lift abruptly disappears when entering a stall, wouldn't people notice a considerable loss in perceived body weight at least as long the aircraft accelerates downward and builds up vertical speed downward.
I expect that the airframe experiences enormous drag in the vertical direction once downward speed is being built up and it probably reaches an equilibrium condition limiting its fall rate.
But if lift disappears abruptly during the stall, then you should have a sensation of free fall (no weight) at least during a short time.
Is this the case or does lift diminish continiously during a stall?
PS: While writing the question i have searched and found those Lift-AoA diagrams (like this: https://www.grc.nasa.gov/www/k-12/airplane/incline.html) that should be able to answer the question.
Basically my question is: How quickly does Lift drop off to the right of the diagram. All diagrams i found did not in fact go much beyond the stall condition.

This article might help give you a better understanding of stalls. Long story short: you won't fall out of the sky.

"wouldn't people notice" you'll be surprised to know what people do not notice. To the point, there is quite some literature (and accidents) about senses deceiving pilots.

@Federico indeed and there is quite a recent video in Skybrary (can be found here ) explaining that problem, called sensory illusions

Lift does not disappear at the onset of a stall. Find a pilot to take you up and demonstrate a stall, in a lot of aircraft it is a gentle break and simple recovery. This is one of the things I don't agree with in the new airman certification standards, they teach recovery before the stall (fine) but never fully stall the airplane.

It's worth noting that a Cessna stalls in a very different manner to a large swept-wing aircraft

Fully stalled training recovery for airliners is being trained already, it's an add-on feature to existing simulators.

Also: A brick does not have wings. If the wings don't give you lift, they still give you drag. So I guess: "It never falls like a brick"

You make some valid observations: not all lift disappears after the stall, and at the event of stall there is a downward acceleration that may or may not be perceived.

Let's start with what happens after the stall. Measurements on airfoils have been taken over 180°, and they show that after the stall, lift drops back until flat plate lift is achieved at the same angle of attack. Increase AoA further, and lift increases again. An example for a relatively thin airfoil from this document:

Pull the aircraft back far enough to 45°, and the $C_L$ is even higher than when it stalled! Of course, the drag will be huge compared to normal operating circumstances, and any fixed wing other than a military jet fighter or a VTOL won't have enough power installed to fly at such angles of attack.
Isn't there a perceived loss of body weight during the acceleration to stall. Yes there is, but it does not last long, is not necessarily very powerful, and could be referenced to other factors than stall, such as turbulence. Once the $C_L$ - $\alpha$ curve hits the trough at 15°, the acceleration stops and weight perception returns to normal. And it's sensory perception, the body only really understands what is happening if all sensory organs are aligned (vision, motion, sound), and can only react in a composed manner if it anticipated the cues and is trained in how to respond. Otherwise we just fight or flee.

You mention the case of AF447. It was in a fully developed stall and descending with on average 10,000 ft/min, but the onset to stall was different. It was  very gradual, and it was a high speed high altitude stall: there simply is not enough air up there to provide enough lift for the weight of an airliner. It can stall at AoA of zero, then start to fall, as a result of which the AoA changes but the pitch attitude of the aircraft does not, and your body is still aligned with gravity in a dark cockpit where the horizon cannot be seen. No physical cues on what is happening whatsoever!

I don't think stalling at AoA of zero is possible. By definition the loss of lift happens when the AoA goes above the threshold, or is near the threshold and we get icing, etc. But you can have insufficient lift if you are too slow/high. You will then fall and AoA will increase even without any nose-up.

As an addendum to Koyovis' answer as concerns AF447:
If you look at some of the parameters below, taken from the BEA accident report the second graph shows the normal acceleration (up and down) in G's. As you see, during the initial upset the passengers would have felt some spikes in G's up to about 1.75G upward and 0.5G downward. This is in the range of heavy to severe turbulence. They would notice. Anyone standing up might even have been tossed around. After that it settles down to around 0.75 to 1.25 G's. So there was no feeling of freefall.
Several places I've read assert that the passengers probably did not know anything was wrong. I strongly disagree with that. Aside from the G forces during the initial part of the accident, the pitch attitude fluctuated between 20° nose up and 10° nose down. The aircraft was also rolling back and forth, sometimes up to 30°. Since it was night IMC they may not have known the seriousness of what was happening, but I can't imagine those kinds of pitch and roll motions would seem normal to anyone.

That first patch of 1.5 g combined with pitch angles of 15 deg, was that directly after the AP disengaage when the pilot pulled on the stick?

@Koyovis Yes. That initial pitch up is the first 20 seconds after AP disconnect